You can use either ‘plain’ Emacs Info or the function octave-help
as your Octave info reader (for ‘help -i’). In the former case,
use info_program ("info-emacs-info").
The latter is perhaps more attractive because it allows to look up keys
in the indices of several info files related to Octave (provided
that the Emacs variable octave-help-files is set correctly). In
this case, use info_program ("info-emacs-octave-help").

If you use Octave from within Emacs, it is best to add these settings to
your ~/.emacs-octave startup file (or the file pointed to by the
Emacs variable inferior-octave-startup-file).